On 05/14/2014 07:11 PM, Thomas Gleixner wrote: > On Wed, 14 May 2014, Carlos O'Donell wrote: >> On 05/14/2014 05:22 AM, Peter Zijlstra wrote: >>>>> I believe the thinking goes that if we get to here, then the lock is in an >>>>> inconsistent state (between kernel and userspace). I don't have an answer >>>>> for >>>>> why pausing forever would be preferable to returning an error however... >>>> >>>> What error would we return? >>> >>> EDEADLK is a valid user return for pthread_mutex_lock() as per: >>> >>> >>> http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html >> >> How is that correct? It isn't a deadlock we've detected but inconsistent >> state between glibc and the kernel. In this case glibc should assert. >> Delaying indefinitely with pause() never seems correct (despite that being >> what we do today). > > If there is inconsistent state detected then the kernel will return > -EPERM or -EINVAL. So lets put inconsistent state aside.
OK. > In glibc you only can detect the simple AA dead lock, i.e lock owner > tries to lock the lock it owns again. Trivial, right ? Agreed. > But glibc has no idea which lock chains are involved and might lead to > a dead lock caused by nested locking, simplest and most popular being > ABBA. OK. > The kernel can (if the implementation is fixed, patch is available > already) very well detect ABBA and even more complex nested lock > deadlocks. So it rightfully returns -EDEADLK and that is completely > correct versus the spec and the call site can do something about it. OK. > And that's not different from the glibc detected AA deadlock at > all. It's just detected by a different mechanism. OK. > On kernel side we currently provide this service only for the PI > futexes because we have a kernel side state representation as long as > the user space state is not corrupted. OK. > Back then when it was implemented the dead lock detection actually > worked and was agreed on by both sides - kernel and glibc - to be > usefull and essential to the whole endavour. I agree that ignoring the situation of corrupted or inconsistent state we should be returning EDEADLK to userspace. We'll cleanup glibc. Cheers, Carlos. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
